/**
 knakul853
 */
class Solution {
public:
    vector<int>ans;
    vector<int> postorderTraversal(TreeNode* root) {

        vector<int>ans;
        stack<TreeNode*>st;
        while(!st.empty() || root )
        {
            if( root )
            {
                st.push(root);
                ans.push_back(root->val);
                root = root->right;
            }

            else{
                root = st.top();st.pop();
                root = root->left;
            }

        }

       reverse(ans.begin(), ans.end());
        return ans;
    }
};

class Solution {
public:
    vector<int>ans;
    vector<int> postorderTraversal(TreeNode* root) {

        if( !root ) return {};

        postorderTraversal(root->left);
        postorderTraversal(root->right);

       ans.push_back(root->val);


        return ans;
    }
};